This invention relates to radiation surveys and, in particular, a method for optimizing the speed of conducting a radiation survey of a surface so that the survey is done with of a high reliability of distinguishing true radiation sources from background.
Currently radiation surveys are performed by technicians with radiation detection equipment, wherein the technician listens to disintegration counts and attempts to determine when a change in count indicates a radioactive source, as contrasted to detector background. The technician physically scans an area, stopping at different points to set the detector or probe down and count for a predetermined time interval, following which a chart or map of surface activity is compiled. However, it is very difficult to reliably distinguish background from radioactive sources at low disintegration counts. In such circumstances, the only way to ensure reliability is to scan the site over a long period of time, i.e., either scan with a safely low velocity, or stop repeatedly at different surface locations. Regulatory requirements of the NRC require detecting the presence of radioactive sources with a 95% or greater reliability, while detecting false sources at a rate of 5% or less. This 95-5 ratio is the industry standard "minimum detectable activity," which is difficult to meet in low count situations.
Prior to this, surveys have necessarily taken a great amount of technician time, since the survey scanning has been done at arbitrarily low speeds in order to meet the probability requirements. Significant development has occurred in the area of automatic survey data collection, transmission and mapping. See U.S. Pat. Nos. 5,416,321; 5,025,150; and 4,924,450. However, there remains a crucial need to cut down the time for taking a reliable survey. It is clear that there would be, for any surface having a given radioactivity concentration or count expectation, an optimum speed which would be slow enough to achieve the desired reliability, but also fast enough to avoid excessive survey time while maintaining such reliability. The need in the art thus has been to provide an accurate and cost effective way of determining an optimum scanning speed which enables minimization of survey time while ensuring the required reliability.